This application claims the priority of British patent application 0904910.7, filed Mar. 23, 2009 and British patent application 1004361.0, filed Mar. 17, 2010, the disclosures of which are expressly incorporated by reference herein.
The present invention relates to apparatus and methods for shimming a magnetic field. In applications such as magnetic resonance imaging (MRI), it is necessary to provide a very homogeneous background magnetic field. For example, a magnetic field of flux density 0.1 T or more must have an inhomogeneity of about 40 parts per million or less peak to peak over an imaging volume of, for example, a 50 cm diameter sphere.
Conventionally, small pieces of ferromagnetic material, such as sheet mild steel, are strategically arranged in calculated positions around the imaging volume to compensate for inhomogeneity in the magnetic field produced by the magnet, in a process known to those skilled in the art as “passive shimming”. For example, a typical MRI magnet may be cylindrical in shape, formed of coils of superconducting wire and housed within a cylindrical cryogen vessel, itself housed within a hollow cylindrical outer vacuum chamber (OVC) which thermally isolates it from ambient temperature. Within the bore of the outer vacuum chamber is positioned a cylindrical gradient coil assembly. This is typically a moulded artefact containing resistive coils within a potting material such as an epoxy resin, and is used to produce orthogonal magnetic field gradients. The resistive coils include gradient coils themselves, and radially outside the gradient coils, gradient shield coils may optionally be provided to reduce the magnitude of magnetic field from the gradient coils reaching the outer vacuum container (OVC). Within the moulded artefact are provided shim slots. These are holes, typically of rectangular cross-section, and typically provided between the gradient coils and the gradient shield coils. Shim trays, of similar rectangular cross-section, are located within the shim slots. Each shim tray contains a number of shim pockets along its length. Pieces of sheet ferromagnetic material, called shim plates, typically mild steel with reproducible magnetic properties, such as that used in transformer laminations, are placed within the shim pockets of the shim trays, and the shim trays loaded into the gradient coil assembly. The pieces of ferromagnetic material affect the magnetic field produced by the magnet, and may be used to improve the homogeneity of the resultant magnetic field. A shim algorithm is used to calculate the number and distribution of the shim plates required to reduce the inhomogeneity of the magnetic field within the imaging volume to the desired level. The shim trays may also or alternatively be placed between the radially outer surface of the gradient coil assembly and the bore of the OVC, or between the radially inner surface of the gradient coil assembly and a body (RF) coil within the bore of the gradient coil assembly.
Shimming conventionally proceeds as follows. A magnet is initially brought to field, and the magnetic field variation is measured over the imaging volume, typically using an array of nuclear magnetic resonance (NMR) probes. Bringing the magnet to field involves gradually increasing electric current flowing through the superconducting coils, a process known as ramping-up. The ramping-up process takes time, and consumes cryogen coolant, as heating occurs within the cryogen vessel. In addition to the time spent ramping, which is typically at least half an hour, potentially several hours, the magnet must be allowed to reach equilibrium, which takes a further one to two hours.
Once the magnetic field variation has been measured, which may be performed using an NMR field camera to map the flux density on the surface of a sphere and decompose this into a sum of spherical harmonics to describe the inhomogeneity, known algorithms may be used to calculate a suitable distribution of shim plates to improve the homogeneity of the magnetic field within the imaging volume. The current in the superconducting magnet is then removed. This “ramping-down” procedure consumes time, and cryogen, similarly to the ramping-up procedure described above. When the magnet has been ramped down, the shim trays are removed from the gradient coil assembly; shim plates are placed in calculated positions in the shim pockets in the shim trays. The shim trays are then replaced in the gradient coil assembly.
The shim plates cannot be loaded or removed at field for safety reasons: significant forces, of hundreds of newtons, are experienced as the shim plates move through a steep magnetic field gradient at the open ends of the bore of a cylindrical magnet. Some experiments have been done on removing and replacing the shim trays with the magnet “at field”, but these have proved less than satisfactory.
The process of ramping up and measuring the magnetic field homogeneity is then repeated. It is unlikely that such shimming will achieve an adequately homogeneous magnetic field in a single iteration due to small errors in the accuracy of positioning of individual shim plates. Typically, two or three shim iterations are required, needing three or four ramping up and two or three ramping down procedures. This is time consuming and wasteful of cryogen coolant.
The present invention aims to provide methods and equipment for effectively shimming a magnetic field produced by a magnet, particularly a superconducting magnet, without the need to ramp down the magnet between shim operations. Avoiding repeated ramping cycles saves time on installing or re-commissioning a superconducting magnet, and reduces consumption of cryogen. Quenches, when a superconducting magnet reverts to a resistive state and loses its stored energy as heat into the cryogen, typically occur only during ramping. By reducing the need for ramp cycles, the likelihood of a quench is reduced by the present invention. The method and apparatus of the present invention is applicable to cylindrical superconducting magnets.
The present invention accordingly provides methods and apparatus as defined in the appended claims. In particular, the present invention provides apparatus and methods for shimming a magnetic field by moving shim pieces within shim channels positioned around a magnetic field to be shimmed. No shim pieces are added to, or removed from, the shim channels during the shimming operation. As no shim pieces need to traverse the steep magnetic field gradient at the open ends of the bore of a cylindrical magnet, the shimming operation may be carried out with the magnet at field. It is unnecessary to ramp the magnet down at all during shimming, according to the methods of the present invention.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.